I think this link explains it exactly:
https://www.dmv.ca.gov/portal/dmv/?1dmy&urile=wcm
ath:/dmv_content_en/dmv/pubs/vctop/vc/d11/c7/a1/22358.4
Seems to be a lot of officer decision on this stuff, especially in CA. I like the way AZ does it with their school signs that have a flashing light that signals when the lower speed is in effect. There is one on the road that leads to the PIR speedway and every year HSR would have a race there you would always see race transporters or out of state pickups pulled over on the Friday when getting to the track!